I believe in magic, the lochness monster and the theory of atlantis.
But, I don't believe in 16MB cache providing a noticeable difference in real world performance over 8MB cache.
He bought 1 harddrive. Your examples state that "you" see an increase in performance with 16MB > 8 MB transferring between hard drives. That's not the case for someone with 1 HDD.
BTW... I also believe in Santa Claus, The Easter Bunny and The Tooth Fairy.
In the end the cache really only helps when transferring large files...I however have over 50gb's of music and countless other anime that I transfer and I believe that the extra few bucks is worth it.
With 16mb the average minimum transfer rate is almost doubled, and the average maximum is increase around 4%.
No transferring files from one storage to the next even...the average minimum is almost doubled from around 18 to 33 on a WD 7200. Then average max from 56 to 58...so not much max but the minimum is a huge increase and I believe worth the money.